Liquid crystal devices of TN (twisted nematic) type, STN (super twisted nematic) type, etc., have conventionally adopted electrode plates comprising ITO (indium tin oxide) films, etc., as transparent electrodes formed on glass substrates in many cases.
The above-mentioned conventional transparent electrode (ITO) has a relatively large resistivity so that it is liable to cause a problematic delay in transmission of applied voltage waveforms according to recent demands for a larger display size and a higher resolution. Particularly, in a liquid crystal device using a ferroelectric liquid crystal requiring a smaller substrate gap or liquid crystal layer thickness on the order of 1.0-2.0 xcexcm, the transmission delay of an applied voltage waveform is liable to be noticeable. A reduced resistance may be given by use of a thicker transparent electrode, but such a transparent electrode having an increased thickness is liable to exhibit poor adhesion onto a glass substrate and require a longer time for film formation, thus resulting in an increased production cost.
In order to solve these problems, there has been proposed a liquid crystal device equipped with an electrode plate comprising a glass substrate having thereon low resistivity metal electrodes of aluminum, etc., and thereon transparent electrodes of ITO, etc., electrically contacting the metal electrodes. Further, in order to comply with demands for liquid crystal devices with a higher aperture ratio and a high speed responsiveness in recent years, it has been desired to develop a metal electrode material of even lower resistivity.
When using metal electrodes of copper having a lower resistivity than aluminum, it is possible to realize a higher aperture ratio and a higher speed responsiveness, but problems are liable to occur regarding adhesion between a glass substrate and a copper electrode layer and the corrosive nature of copper. As a solution to such problems, there has been proposed an electrode plate having a sectional structure as shown in FIG. 17 including a glass substrate 100 having thereon an adhesion layer 101 of chromium, a principal conductor layer 102 of copper and thereon a chromium-copper alloy layer 103 formed by sintering in a reducing atmosphere. The adhesion layer 101 and the principal conductor layer 102 may be formed by forming a chromium layer and a copper layer respectively by sputtering on the glass substrate, followed by etching in a prescribed electrode pattern.
When providing ordinary print circuit boards, the etching of a copper layer may be performed using an iron chloride solution, a copper chloride solution or a liquid acid mixture principally comprising nitric acid and phosphoric acid as an etchant (or etching liquid), and dipping a substrate coated with a copper layer together with an etching pattern in a vessel of the etchant or showering the etchant onto the substrate to etch the copper layer into a desired pattern.
However, in the electrode plate structure shown in FIG. 17 including a glass substrate 100 and a laminate film disposed thereon comprising a copper principal conductor layer 102 and a chromium adhesion layer 101 for enhancing the adhesion between the copper layer 102 and the glass substrate 100, the chromium layer 101 and the copper layer 102 cannot be patterned in a single etching step. More specifically, when an etchant as described above having an etching effect on both the chromium and copper layers is used for etching of the laminate layer, a cell effect occurs between the chromium and copper layers via the etchant. As a result, the layer of chromium (similar to molybdenum and titanium) having a larger ionization tendency (i.e., a smaller standard electrode potential) than copper is selectively etched to result in an abnormal undercut. The undercut may remain within a tolerable limit for an electrode or wire pattern having a width on the order of mm as in ordinary print circuit boards, but provides a serious problem for providing an electrode or wire pattern having a width on the order of 10 xcexcm. Thus, the undercut can lead to peeling of the laminate electrode pattern.
For the above reason, it is necessary to use an etchant reacting only on the adhesion layer metal and an etchant reacting only on copper and repeat the etching of the respective metal layers while changing the etchant. More specifically, in the case of providing a chromium adhesion layer, it is possible to effect selective etching of the chromium and copper layers in multiple steps using etchants selectively reacting on chromium and copper, but this requires a complicated process and a considerable processing time. Moreover, the chromium-containing etching waste liquid can provide another difficulty with respect to environmental pollution.
On the other hand, in the case of providing an adhesion layer of molybdenum or titanium, most etchants reactive with these metals are also reactive with copper and also cause the undercut problem. Incidentally, the use of a potassium ferricyanide solution allows selective etching of molybdenum, but simultaneously promotes the growth of a stable oxide film on copper. The oxide film functions as a passivation film against a copper chloride solution as a selective etchant for copper, thus preventing the copper etching. An acid solution containing nitric acid, etc., can effect etching of a copper layer coated with the oxide film, but this is also accompanied with the problem of undercut arising from the difference in ionization tendency.
On the other hand, in the dry etching process, such as the reactive etching process, etc., the above-mentioned problems arising from the difference in ionization tendency can be obviated, but the etching speed is too small for commercial application.
As another process for providing a patterned copper film without resorting to an etching process, it has been proposed to form an adhesion layer film of chromium, molybdenum, etc., by sputtering, followed by etching into an electrode pattern, and to selectively coat the patterned adhesive layer with copper by plating and also with a protective layer by plating, etc. This process obviates most of the practical problems mentioned above accompanying copper electrode production, but it is difficult to form an accurate film thickness on the order of xcexcm as required on an electrode plate for a liquid crystal device. Further, the adhesion layer formed by sputtering on the glass substrate shows a good adhesion onto the glass substrate, but shows a rather poor adhesion with a copper plating layer, thus possibly requiring another adhesion promoting layer between the adhesion layer and the copper layer.
From the above discussion, it is still desirable to provide an electrode plate with patterned electrodes by sequentially forming an adhesive layer and a copper layer on a substrate and then etching the layers in a prescribed electrode or wire pattern. In this instance, it is desired to effect simultaneous etching of the adhesive layer and the copper layer in a single etching step.
On the other hand, the wet etching process has been frequently used as an etching process for locally removing or patterning a film formed on a substrate not only for production of semiconductor devices and ornamental articles, because of a simple structure and a low production cost.
FIG. 11 is an illustration of such a wet etching process. Referring to FIG. 11, in this process, a vessel 81, such as a beaker, is charged with an etchant 82, in which a substrate 84 having thereon a pattern of resist 83 exposing portions to be etched is dipped for a prescribed period. After the prescribed time, the substrate 84 is taken out and washed with, e.g., a large amount of flowing water to terminate the etching. In this way, a film on a substrate can be etched into various patterns through a simple step according to the wet etching process.
In the wet etching process, however, a difficulty is sometimes encountered in that the etching speed can vary locally on the substrate depending on the affinity between the etchant and the film to be etched. For example, etching of an aluminum (Al) film may generally be performed using a liquid mixture of nitric acid, phosphoric acid and acetic acid (which may sometimes be called mixture acid aluminum liquid) and, in that case, the central portion of the film on the substrate is preferentially etched because of the difference in etching speed between the central portion and the edge portion on the substrate. On the other hand, etching of a copper (Cu) film may frequently be performed using a liquid mixture of ferric chloride and acetic acid. The mixture liquid provides a greater etching speed at an edge portion than a central portion of the Cu film on the substrate, thus causing a preferential etching at the edge portion.
Further, as the wet etching process proceeds isotropically by way of a chemical reaction between the film and the etchant, it is liable to cause a side etching phenomenon as shown at 90 in FIG. 12 (showing a side view of an etched film portion) wherein a portion below a resist 83 of a Cu film formed on a substrate 86 of, e.g., glass, is etched hemispherically. Because of the side etching, it becomes difficult to effect etching of a minute pattern on a micron order.
In order to obviate such difficulties with respect to etching speed distribution and side etching, there has been proposed a wet etching process utilizing a showering scheme (wherein the etchant is showered onto a substrate). As a result, it has become possible to effect etching of a pattern of several xcexcm, so that the showering process has been frequently used, particularly in an early period of semiconductor device production.
FIG. 13 illustrates such a showering scheme wet etching process. Referring to FIG. 13, an etchant 88 is showered from a number of nozzles 87, and a substrate 84 to be etched is moved under the shower of the etchant 88. As a result, the etchant 88 can be uniformly supplied to the entire area of the substrate 84 surface, and uniform etching over the entire surface becomes possible.
Further, in the showering scheme process, since the etchant 88 may be sprayed with a certain degree of directionality onto the substrate 89 surface, the amount of side etching 90 can be reduced as shown in FIG. 14 (compared with a conventional state shown in FIG. 12 described above), so that it can provide an etching pattern showing a higher aspect ratio.
On the other hand, in the production of semiconductor devices, a dry etching process has become popular due to high accuracy controllability and occurrence of less waste liquid in addition to the above-described demands for patterning of a laminate film of different materials and an etching pattern giving a high aspect ratio.
In the dry etching process, a film on a substrate is vaporized through a plasma reaction with a reactive gas to remove the film from the substrate. The dry etching process requires a reaction vessel to be placed under vacuum and a huge etching apparatus for processing of a substrate having a diameter of several tens cm. Accordingly, this process cannot be readily applicable to a larger size liquid crystal substrate required to meet the demand for larger size liquid crystal panels in recent years.
In semiconductor device production in recent years, the use of Cu films has been examined for providing lower resistivity electrodes or wires, but no reactive gases for providing a vaporizable copper compound have been discovered, hence, the dry etching process has not been successfully adopted therefor. Further, the dry etching process is accompanied with a problem that the substrate is liable to be damaged due to irradiation with charged particles.
For obviating the difficulties of the dry etching process, the wet etching process is again receiving better evaluation in recent years. Further, the problem of side etching which has been regarded as a defect of the conventional wet etching process can be solved by formation of a side wall protective film, thus allowing etching of a higher aspect ratio.
FIG. 15 illustrates a principle of the side wall protection film. A Cu etching process is explained for example. A mixture aqueous solution of ferric chloride and acetic oxide is generally used as the etchant, and ca. 1% of thiourea is added thereto. The etchant is sprayed onto a substrate 84 surface. As a result, a reaction according to the following formula (1) is caused at the Cu film 85 surface to form a deposition of insoluble matter 89, which inhibits the etching: 
As shown in FIG. 15, the etchant 88 is sprayed onto the substrate 84 surface so that the bottom portion rather than the side wall portion of the Cu film 85 is preferentially eroded to provide a larger etching speed in a downward direction, thereby allowing a higher aspect ratio of etching than before.
On the other hand, known etching apparatuses include one wherein an etchant is sprayed onto a rotating substrate to be etched, and the showering scheme can be adopted for providing better controllability.
FIG. 16 illustrates such a spin coating apparatus adopting the showering scheme. Referring to FIG. 16, in the apparatus, a substrate 91 to be etched held on a rotating substrate holder 90 is rotated together with the holder 90. After the rotation of the holder 90 is stabilized, a nozzle 92 disposed above the substrate 91 is vibrated, and a changeover valve 93 is turned on to open an etchant pipe 94, thus spraying the etchant through a nozzle 92 onto the substrate 91 surface.
After etching for a prescribed period, the changeover valve 93 is switched to stop the etchant and open a pipe 95 for a rinse such as water, thus spraying the rinse onto the substrate 91 surface to wash off the etchant 94. The substrate rotation speed and the vibration speed of the nozzle during the etching may be appropriately adjusted depending on the combination of etchant and film to be etched.
In such a spin etching process wherein an etchant supplied to a substrate surface is liberated from the substrate due to a centrifugal force accompanying the substrate rotation, there are attained several advantages for providing an improved etching accuracy not attainable by the conventional wet etching process. These advantages include: a fresh etchant is less liable to be buffered with the etchant after the reaction; the etchant can be supplied over the entire substrate by an appropriately adjusted combination of substrate rotation speed and nozzle vibration frequency; and the etching time can be controlled with improved accuracy by shortening the distance between the etchant discharge nozzle and the etchant selection valve.
However, one difficulty with the spin etching process is that the etching termination point determination accuracy is inferior as compared to the dry etching process.
More specifically, in the dry etching process using a plasma reaction for etching, the reaction of the film on the substrate for gassification is accompanied with light emission in many cases, and the resultant emission spectrum is peculiar to the film material and the etching gas. Accordingly, by specifying a light emission spectrum and measuring an intensity change of the spectrum, it is possible to accurately evaluate the progression state of the etching.
In contrast thereto, in the wet etching process, no noticeable phenomenon like light emission is involved. Accordingly, the progression of the etching has been evaluated according to an etching time experimentally determined based on trial experiments performed in advance. However, the evaluation of the progression state according to the etching time requires an accurate stabilization of reaction speed as a critical factor so that it also becomes necessary to accurately control the etchant and the temperature of the substrate to be etched.
In this regard, a large amount of etchant is used in the wet etching process, and, in order to suppress the etching cost and minimize environmental pollution, it has been generally practiced to recover the etchant and repeatedly use the etchant. However, during such repeated use of the etchant, the etching speed can be lowered due to saturation of a reaction called xe2x80x9cetchant fatiguexe2x80x9d or the vaporization of a relatively volatile component in the case of an etchant comprising a mixture of several components which causes a compositional change of the etchant, the latter can even lead to an increase in etching speed.
Thus, even if accurate control of the etchant and the substrate temperature are attempted so as to provide an accurately stabilized reaction speed, it becomes difficult to effect stable etching control with good reproducibility due to factors, such as etchant fatigue, and also to evaluate etching progression with good accuracy.
Accordingly, an object of the present invention is to provide an electrode plate having thereon an electrode laminate of a metal adhesion layer and a principal conductor layer of copper which can be patterned with high accuracy through a single etching step, a process for producing such an electrode plate, a liquid crystal device equipped with such an electrode plate, and also a process for producing such a liquid crystal device.
Another object of the present invention is to provide a process and an apparatus for spin etching allowing accurate determination or evaluation of etching progression.
According to the present invention, there is provided an electrode plate comprising a substrate and a plurality of patterned electrodes formed on the substrate, wherein each patterned electrode has a laminate structure including a first layer of nickel metal formed on the substrate and a second layer of copper formed on the first layer.
According to the present invention, there is provided a process for producing an electrode plate sequentially comprising a first layer-forming step of forming a first layer of nickel metal on a substrate, a second layer-forming step of forming a second layer of copper on the first layer, and an etching step of spraying an etchant downwardly onto the first and second layers on the substrate while rotating the substrate, thereby etching the first and second layers in a prescribed pattern to form a plurality of laminated metal electrodes.
According to another aspect of the present invention, there is also provided a spin etching process, comprising the steps of fixing a substrate to be etched onto a substrate holder, and spraying an etchant onto the substrate while rotating the substrate holder to etch a surface of the substrate, wherein etching progression is monitored by illuminating the substrate with light, detecting a light quantity or a spectrum pattern of emitted light as reflected light or transmitted light from the substrate, and evaluating the detected emitted light from the substrate.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.